1. Field of the Invention
The present invention relates to an internal combustion engine system switchable between a spark ignition operation and a homogeneous charge compression ignition operation.
2. Description of the Related Art
In recent years, in order to improve fuel efficiency and reduce emission matters in internal combustion engines, compression ignition internal combustion engines represented by homogeneous charge compression ignition internal combustion engines have been under review. In a homogeneous charge compression ignition internal combustion engine, oxygen containing gas and a compression ignition fuel are introduced into a cylinder and compressed to ignite spontaneously.
However, as distinct from spark ignition internal combustion engines, homogeneous charge compression ignition internal combustion engines have problems that it is difficult to control ignition timing and the engines can be operated safely only in a narrow operating range. More specifically, the problems mean that use of a fuel with high ignitability tends to cause knocking when required load of the engine is high, and that use of a fuel with low ignitability tends to cause misfire when required load of the engine is low. In order to overcome the problems, there is known an internal combustion engine system switchable between a spark ignition operation and a homogeneous charge compression ignition operation depending on operation conditions or the like (for example, see Japanese Patent Laid-Open No. 2001-152919, and No. 2002-130006.)
Furthermore, in order to overcome the problems, it is conceivable that two or more types of fuels having different ignition characteristics are prepared, and the mixture ratio of the fuels is controlled when the fuels are provided to the homogeneous charge compression ignition internal combustion engine, thereby controlling the ignition timing. Examples of the two or more types of fuels having different ignition characteristics may include a combination of liquid hydrocarbon such as gasoline or GTL naphtha and ethanol.
Use of ethanol allows the so-called “carbon neutral effect”, thereby contributing to carbon dioxide emission reductions. The carbon neutral effect means that combustion of ethanol derived from plants emits carbon dioxide but the net carbon emissions are zero because the material plants absorb carbon dioxide by carrying out photosynthesis in their growth processes.
The liquid hydrocarbon and the ethanol may be contained in separate tanks. Alternatively, the liquid hydrocarbon and the ethanol may be contained as a mixed fuel in a single tank and separated into the liquid hydrocarbon and the ethanol as necessary. It is known that addition of water to the mixed fuel of liquid hydrocarbon and ethanol easily separates the fuel into the liquid hydrocarbon and an ethanol-water mixture (for example, see Japanese Patent Laid-Open No. 58-96155).
When the liquid hydrocarbon is compared to ethanol, ethanol is less self ignitable than the liquid hydrocarbon. Therefore, a mixture ratio of the ethanol and the liquid hydrocarbon is adjusted, thereby changing ignition characteristics, for example, represented by the octane number. As a result, the ignition timing of a homogeneous charge compression ignition can be controlled.
However, there is demanded development of an internal combustion engine system in which the ignition timing of a homogeneous charge compression ignition can be controlled more effectively.